Fluid heating system for processing semiconductor materials

ABSTRACT

A system for heating solvents in processing semiconductor wafers has a coiled solvent tube, a coiled cooling water tube, and electric heater elements, cast in place within an aluminum casting. The solvent flows through the solvent tube and is heated by conduction of heat through the casting. The solvent is safely isolated from the heating elements. Water is pumped through the cooling water tube, to cool the casting if solvent flow is interrupted, or if the measured casting temperature exceeds a predetermined set point temperature. Solvent temperature is maintained by controlling power to the heating elements based on the measured solvent temperature at the processing chamber.

This application is a continuation of Ser. No. 09/372,849 filed Aug. 12,1999, now U.S. Pat. No. 6,536,450, which is a continuation-in-part ofSerial No. 60/142,864, filed Jul. 6, 1999, both incorporated herein byreference.

The field of this invention is automated processing systems used forprocessing semiconductor wafers, hard disk media, substrates, andsimilar flat media requiring low levels of contamination. The inventionalso relates to heaters for solvents and other flammable fluids.

BACKGROUND OF THE INVENTION

Computers, televisions, telephones and other electronic products containlarge numbers of electronic semiconductor devices. To produce electronicproducts, hundreds or thousands of semiconductor devices aremanufactured in a very small space, using lithography techniques onsemiconductor substrates, such as on silicon wafers. A large number ofindividual processing steps may be required to manufacture thesemiconductor devices. Various machines and methods have been developedfor these applications. For example, U.S. Pat. No. 6,279,724,incorporated herein by reference, describes a system having processingchambers for processing and cleaning flat media (referred to below as“wafers”).

In certain processing steps, it is advantageous, or necessary, to applysolvents to the wafers. To speed up and to better control the waferprocessing, it is desirable to heat the solvent, and to closely controlthe temperature of the solvent which is applied to, e.g., sprayed onto,the wafers.

Heating solvents in a safe and reliable way presents unique challenges,because many solvents are combustible. Conventional heating techniquesused for other types of fluids are generally unacceptable for heatingsolvents, due to the risk of igniting the solvent by a malfunctioningelectrical heater or heater controller; or because they are unsuitablefor the semiconductor manufacturing environment, which must be extremelyclean and free of particles; or because they cannot meet the duty cyclerequirements needed in semiconductor manufacturing. Quartz heaterelements, which have been used to heat various liquids used insemiconductor manufacturing, are unacceptable for heating solvents,because of the risk that the brittle quartz will crack or break,exposing the combustible solvent to extreme temperatures and electricalcontacts. On the other hand, tougher materials, such as stainless steelor Teflon, which can reduce or eliminate the risk of breakage of aheating element, and which are also compatible for use with solvents,are unfortunately poor conductors of heat. Accordingly, efficientlyheating solvents has remained as a significant engineering challenge.

In the past, blanket heaters have been provided to heat a solvent in astorage tank to a specific set point temperature prior to delivery ofthe solvent to a wafer processing chamber. The tank blanket heaters arecontrolled to maintain that set point during processing. While thistechnique overcomes the difficulties presented by the volatilecharacteristics of solvents, it has certain disadvantages. Initially, asthe entire tank contents must be heated, the desired temperature changesoccur slowly. In addition, the tank heaters are controlled based on thesolvent temperature in the tank, resulting in significant delays incorrecting the solvent temperature back, to the set point, duringprocessing. The heat up time is also long, due to the conduction heatingthrough the stainless steel tank walls, and due to the large mass ofstationary solvent. As a result, the temperature of the solvent at thechamber cannot be closely controlled, resulting in poor processinguniformity, low strip/removal rates, and longer process times. Typicaltemperature drops are 4-8° C. from a set point of 75° C. In addition,the through put of the system, e.g., in the number of wafers processedper hour, is limited due to the time required for heating the solvent.

Accordingly, there is a need for an improved solvent heater, especiallyfor use in processing semiconductor wafers.

SUMMARY OF THE INVENTION

In a first aspect of the invention, in a semiconductor processingmachine, a heater for heating solvents includes a solvent tube, and acooling tube, extending through a casting or other solid form. One ormore heating elements extend into the casting. Heat from the heatingelement is conducted through the casting or solid form, to heat solventflowing through the solvent tube. As the solvent is isolated from theheating element via the solvent tube and solid material of the castingor solid form, the potential for igniting the solvent during heating isreduced or eliminated.

In a second aspect of the invention, the solvent tube and cooling tubeare shaped into coils, with the cooling coil surrounded by the solventcoil. The cooling coil can rapidly cool the casting, if necessary, andcan also help to control temperature.

In a third aspect of the invention, the heating element has spaced apartlegs which straddle the cooling coil.

In a fourth aspect of the invention, the solvent tube, cooling tube, andheating element are cast in place.

In a fifth aspect of the invention, the casting is enclosed within acontainer. The walls of the container are insulated from the casting. Apurge gas is provided in the space between the insulation and thecontainer walls, to provide an inert atmosphere around the casting.

In a sixth aspect of the invention, cool water is circulated through thecooling tube to rapidly cool the solvent to a temperature low enough toallow the used solvent to be drained from the semiconductor processingmachine into the waste lines of a semiconductor manufacturing facility.

Other features and advantages will appear hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, where the same reference number denotes the sameelement, throughout all of the views:

FIG. 1 is a perspective view of an automated semiconductor processingsystem;

FIG. 2 is a perspective view of the processing unit shown in FIG. 1;

FIG. 3 is a simplified perspective view of the solvent heater shown inFIG. 2;

FIG. 4 is a side view thereof;

FIG. 5 is a top view thereof;

FIG. 6 is a simplified section view taken along line 6—6 of FIG. 5:

FIG. 7 is a front view showing the detailed construction of the solventheater;

FIG. 8 is a side view thereof;

FIG. 9 is a plan view thereof (with various components removed forclarity of illustration);

FIG. 10 is an enlarged detail showing certain of the features of FIG. 9;

FIG. 11 is a top view of the heating elements showing in FIGS. 7-8;

FIG. 12 is a side view thereof; and

FIG. 13 is a schematic illustration of components of the processing unitshown in FIG. 2.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning now in detail to the drawings, as shown in FIG. 1, a waferprocessing system 20 includes a processing unit 22 having two side byside centrifugal processors 24. Each centrifugal processor 24 has achamber or bowl 26. Wafers are placed into a rotor within the chamber26. The rotor spins the wafers, while solvents or other fluids aresprayed or applied to the wafers, during specific processing steps inthe creation of semiconductor devices. The flat media processing system20 is described in detail in U.S. Pat. No. 6,279,724, incorporatedherein by reference. As shown in FIG. 2, a solvent heater 40 is includedin the processing unit 22, to supply heated solvent to a chamber 26.Referring momentarily to FIG. 13, the solvent heater 40 is connected tothe chamber 26 and to a solvent storage tank 28 with fluid connectionlines.

Turning now to FIGS. 3-8, the solvent heater 40 includes a casting 70(preferably aluminum alloy 319) or an equivalent substantially solidform of thermally conductive material. A solvent coil tube 84 and acooling coil tube 80 are both embedded in, or cast in place, within thecasting 70 or other solid form (hereinafter referred to as a casting).The solvent coil tube 84 has a solvent inlet 62 and solvent outlet 60with pipe fittings, preferably compression (Swadgelok) fittings.Similarly, the cooling coil tube 80 has an inlet fitting 64 and anoutlet fitting 66. In the embodiment shown, the solvent coil tube 84 isa ½ inch OD×0.049 wall×209 inches long (13 mm×1.2 mm×5300 mm) No. 316stainless steel tube and the cooling coil tube 80 is a ¼ inch (6 mm)OD×153 inches (3890 mm) long No. 304 stainless steel tube. As shown inFIGS. 10 and 11, the fittings are welded to bosses 68 which are alsocast in place, at the top and bottom of the casting and they become partof the casting.

The casting 70 is formed in a cylindrical shape. The solvent coil tube84 is preferably concentric with, and surrounds the cooling coil tube80. Three electrical resistance heaters 72 are equally spaced apartwithin the casting 70, as shown in FIG. 9. Referring momentarily toFIGS. 11 and 12, each heater 72 has a first or inside leg 74 connectingto a second or outside leg 76 at an elbow 75. The first leg 74 isseparated from the second leg 76 by a space 78.

Referring to FIGS. 7-8, the first leg 74 of each heater is inside of thecooling coil tube 80, while the second leg 76 of each heater 72 isoutside of the cooling coil tube. Both legs 74 and 76 of the heaters 72are within or surrounded by the solvent coil tube 84.

As best shown in FIGS. 7 and 8, the casting 70 is contained within acanister 42. The cylindrical canister 42 has an annular top rim 46. Around top plate 48 is secured to the top rim 46 by fasteners 90. Aninsulating gasket 56 separates the top plate 48 from the top rim 46. Thecasting 70 is attached to the top plate 48 via bolts 95. The casting 70is accordingly suspended within the canister 42, such that the canisterwalls are separated from the casting by an air gap 104, on all sides(except at the top). A jacket of foam insulation 102 surrounds thecasting 70, to reduce heat loss.

The solvent inlet and outlet fittings 62 and 60, and the cooling inletand outlet fittings 64 and 66, pass through and seal externally againstthe canister 42 sidewalls. The canister 42 has removable cover plates45, to allow installation of the casting into the canister.

Referring still to FIGS. 7 and 8, a stainless steel pipe section 50 isalso cast in place and extends through the top plate 48. The pipesection is filled with cast aluminum. Three power leads 54 and a neutrallead from the heating elements 72 extend out of the pipe section 50 andextend into a NEMA box 52 threaded onto to the upper end of the pipesection 50. Electrical connections to provide power to the heatingelements 72 are made within the NEMA box 52. In the embodiment shown,the heating elements are wired in a 3-phase Wye configuration, andoperate at 380V or 480V, with a combined power of about 7.5 kw.

The canister 42 is sealed against its environment via the top plate 48and the gasket 56, the cover plate 45, and the sealing surfaces on thefittings 60, 62, 64 and 68. A purge gas inlet 108 and a purge gas outlet106 extend into the canister 42, so that the air gap 104 can be filledwith another gas, such as nitrogen, which does not support combustion.

A first thermocouple 92 is located at the solvent outlet, to monitor thesolvent tube wall temperature. A redundant thermal couple 95 is providedas a backup also at the solvent tube wall, for use if the firstthermocouple fails. The thermocouples 92 and 95 are attached to outletend of the solvent coil tube 84 and are cast in place. As shown in FIG.9, another thermocouple 96 is cast in place near the heating elements72, to monitor the heating elements temperature. The wire leads from thethermocouples 92, 95 and 96 extend out of the top of the casting 70,through the pipe section 50, and into the NEMA box 52.

A snap switch 100 is located within the NEMA enclosure 52 on the topsurface of the casting 70, within the pipe section 50. The snap switch100 senses the casting temperature and cuts power to the heaters 72, ifa predetermined set point is reached.

Turning to FIG. 13, blanket heaters 34 surround a solvent tank 28. Atank pump 114 pumps solvent from the tank 28 through a flow sensor 116and into the heater 40. Solvent flows through the heater and through afilter 118 to a selection valve 30. The valve directs the solvent toeither the chamber 26 or to a recirculation line 32. A return line 120returns the solvent to the tank 28. As schematically shown in dashedlines in FIG. 13, a computer controller 128 is linked to thethermocouples 92, 94, 96; pump 114; flow sensor 116; valve 30; and topower controls for the heaters 34 and 40, and also to various othersensors and components. Nitrogen is continuously pumped through thecanister via the purge inlet 108 and outlet 106. If solvent or solventvapors collect in the canister 42 due to a leak, the nitrogen purgingreduces any potential for ignition.

For each chamber 24 in the system 20, a minimum of one heated tank 28 isrequired to store the fluid solvent required for the processing. FIG. 13conceptually shows a design for a single chamber 24.

In operation, a solvent fluid temperature set point (e.g., 70° C.) isentered into the controller or other circuitry as a fixed value duringmanufacture of the system 20. For safety and quality assurance reasons,this set point is limited to a maximum of e.g., 90° C. via software inthe controller 38. At start up, power is applied to the blanket heaters34. Solvent is pumped through the solvent coil tube 84. The flow ofsolvent, as detected by the flow sensor 116, enables power to theheating elements 72, which heat the casting. Temperatures are monitoredvia the thermocouples 92, 94 and 96. The solvent flow through thesolvent coil tube 84 is preferably turbulent, to increase conductiveheat transfer from the heating elements 72, through the casting, andinto the solvent.

The valve 30 is positioned to direct the flowing solvent through therecirculation line 32 and back to the tank 28. When the solvent in thetank has reached the set point temperature, the processor 24 is ready toprocess wafers. At appropriate times during the process cycle, the valve30 is positioned to direct solvent to the chamber 26. The solvent issprayed onto wafers spinning in a rotor within the chamber 26. Thesolvent loses heat and cools down. The cooled solvent is then collectedand flows under gravity through the return line 120 to the tank 28.

The power to the heating elements 72 is controlled based on thetemperature of the solvent entering the chamber 26. This allows forrapid adjustments, so that the variations from the set point are greatlyreduced.

Solvent is safely heated, as the solvent is separated from the heatingelements 72 by the solid barrier of the cast material separating theheating elements and solvent coil tube 84. In the event of an overtemperature condition, or if solvent stops flowing through the solventcoil tube 84, water is pumped through the cooling coil tube 80, toremove heat. The computer controller 38 linked to the water valve 110opens valve 110 if a failure is detected. When the water valve 110opens, water flows through the cooling tube 80, to cool the casting 70.The water then flows out to a drain.

With a solvent tank 28 having a volume of 15 liters, test data showsthat the solvent temperature can be raised from 28° C. to 70° C. in 7-8minutes, and from 28° C. to 87° C. in about 11 minutes, using acontinuous recirculating flow rate of about 11 liters per minute. Thetemperature increase is about 5° C. per minute.

In many semiconductor fabrication facilities, waste line pipes cannotaccept fluids warmer than about 50° C., due to the pipe material, andthe chemically reactive characteristics of certain waste fluids,including solvents. Accordingly, fluids, such as solvents which areheated to e.g., 75° C., as is needed for efficient processing, cannot bereleased into waste lines, without first allowing them to cool down.Ordinarily, heated solvents are allowed to cool in a tank within aprocessing unit, such as the tank 28 in the processing unit 22. However,the processing unit 22 is then not useable during the cool downinterval. Consequently, manufacture of semiconductors is slowed. Theheater 40 allows this drawback to be minimized, by actively cooling thesolvent, instead of storing the solvent in bulk and waiting for it topassively cool down in the tank. Specifically, to cool the solventrapidly to a temperature acceptable for release into manufacturingfacility waste lines, the solvent is circulated through the heater 40.However, the heating elements are turned off and cold water iscirculated through the cooling coil. As a result, the used solvent israpidly cooled and can be promptly released into the facility wastelines. The processing unit 22 is then available to process additionalflat media.

Thus, a novel solvent heater for use with a semiconductor processingsystem safely heats solvents, decreases initial heat up time, bettermaintains target solvent temperature, and reduces recovery time. Variouschanges, modifications, and substitutions of equivalents may of coursebe made, without departing from the spirit and scope of the invention.The invention, therefore, should not be restricted, except by thefollowing claims, and their equivalents.

We claim:
 1. A heater for heating a process liquid, comprising: aprocess liquid coil within a metal block; a cooling water coil withinthe metal block, at least one heating element in the metal block; acontainer surrounding the metal block; and a purge gas space between thecontainer and the metal block.
 2. The heater of claim 1 furthercomprising a layer of insulation around the metal block.
 3. The heaterof claim 1 further comprising a container surrounding the metal block,wherein inner walls of the container are separated from the metal blockby an air gap.
 4. The heater of claim 1 wherein the process liquid coilsurrounds the cooling water coil.
 5. The heater of claim 1 wherein theprocess liquid coil is concentric with a longitudinal axis of thecooling water coil, and where a plurality of heating elements areequally spaced apart around the longitudinal axis of the cooling watercoil.
 6. The heater of claim 1 wherein the heating element includes afirst leg within the cooling coil and a second leg outside of thecooling coil.
 7. The heater of claim 1 wherein the metal block comprisesa casting and the heating element is cast in place within the metalcasting.
 8. The heater of claim 1 wherein the process liquid coil isconcentric with a first axis, and wherein the heating element extendsparallel to a second axis perpendicular to the first axis.
 9. The heaterof claim 1 wherein a plurality of elongate heating elements are providedextending substantially from a first end to a second end of the block ofmetal.
 10. A heater for heating a solvent in a wafer processing system,comprising: a block of metal; a solvent coil extending through the blockof metal and having an inlet connectable with a solvent supply and anoutlet connectable with a processing chamber in the processsing system;a cooling coil extending through the block of metal and having an inletconnectable to a cooling water supply; at least one heating element inthe block of metal; a container around the block metal with a purge gasspace between the container and the block of metal; and a purge gassupply port connecting into the purge gas space.
 11. A heater forheating a solvent in a wafer processing system, comprising: a block ofmetal; a solvent coil extending through the block of metal and having aninlet connectable with a solvent supply and an outlet connectable with aprocessing chamber in the processsing system; a cooling coil extendingthrough the block of metal and connectable to a cooling water supply;one or more heating elements in the block of metal; a container aroundthe block metal with a purge gas space between the container and theblock of metal; a pipe section attached to a first end of the block ofmetal; and a box attached to the pipe section, with electrical leadsfrom the heating elements extending out of the block of metal, throughthe pipe section and into the box.